1. Field of the Invention
The present invention relates to a method for writing servo onto disks of a hard disk drive.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.
Information is typically stored in radial tracks that extend across the surface of each disk. Each track is typically divided into a number of segments or sectors. The voice coil motor and actuator arm can move the heads to different tracks of the disks.
FIG. 1 shows a typical track that has a number of fields associated with each sector. A sector may include an automatic gain control (“AGC”) field 1 that is used to establish a timing reference for the circuits of the drive, and ID 3 and Gray Code 4 fields to provide sector and track identification.
Each sector may have also a servo field 5 located adjacent to a data field 6. The servo field 5 contains a plurality of servo bits A, B, C and D that are read and used to position the head 7 relative to the track. The fields 1-5 must be written onto the disk surfaces during the manufacturing process of the disk drive. These fields are typically written with a servo writer. The servo tracks are sometimes written using a number of spiral servo tracks initially written onto the disks. FIG. 2 shows an example of a disk with number of spiral servo tracks. Using spiral servo tracks is sometimes referred to as an Ammonite servo write process. The spiral servo tracks are used to write the final radial servo tracks that are utilized during the normal operation of the disk drive. This process is described in U.S. Pat. No. 5,668,679 issued to Swearingen et al.
As shown in FIG. 3, the disk has a plurality of spiral servo patterns incrementally spaced across the surface of the disk. To write servo the servo writer reads the disk to detect a spiral servo signal. The servo writer uses the detected spiral servo signal to time the writing of a permanent A, B, C and D servo burst pattern.
The write heads can be moved by a push-pin that pushes an actuator attached to the heads. The heads of the servo writer are accelerated from a rest position to a constant velocity. The servo writer does not begin to write the spiral servo pattern until the head reaches a constant velocity. This results in an “acceleration” zone of the disk that does not contain servo. It is possible to write a final 4-burst servo pattern in the acceleration zone but because of disk run-out the servo is not very accurate. It would be desirable to fill the acceleration zone of the disk with accurate 4-burst servo patterns.